BlitSaw.cpp

/***************************************************/
/*! \class BlitSaw
    \brief STK band-limited sawtooth wave class.

    This class generates a band-limited sawtooth waveform using a
    closed-form algorithm reported by Stilson and Smith in "Alias-Free
    Digital Synthesis of Classic Analog Waveforms", 1996.  The user
    can specify both the fundamental frequency of the sawtooth and the
    number of harmonics contained in the resulting signal.

    If nHarmonics is 0, then the signal will contain all harmonics up
    to half the sample rate.  Note, however, that this setting may
    produce aliasing in the signal when the frequency is changing (no
    automatic modification of the number of harmonics is performed by
    the setFrequency() function).

    Based on initial code of Robin Davies, 2005.
    Modified algorithm code by Gary Scavone, 2005.
*/
/***************************************************/

#include "BlitSaw.h"

namespace stk {
 
BlitSaw:: BlitSaw( StkFloat frequency )
{
  if ( frequency <= 0.0 ) {
    oStream_ << "BlitSaw::BlitSaw: argument (" << frequency << ") must be positive!";
    handleError( StkError::FUNCTION_ARGUMENT );
  }

  nHarmonics_ = 0;
  this->reset();
  this->setFrequency( frequency );
}

BlitSaw :: ~BlitSaw()
{
}

void BlitSaw :: reset()
{
  phase_ = 0.0f;
  state_ = 0.0;
  lastFrame_[0] = 0.0;
}

void BlitSaw :: setFrequency( StkFloat frequency )
{
  if ( frequency <= 0.0 ) {
    oStream_ << "BlitSaw::setFrequency: argument (" << frequency << ") must be positive!";
    handleError( StkError::WARNING ); return;
  }

  p_ = Stk::sampleRate() / frequency;
  C2_ = 1 / p_;
  rate_ = PI * C2_;
  this->updateHarmonics();
}

void BlitSaw :: setHarmonics( unsigned int nHarmonics )
{
  nHarmonics_ = nHarmonics;
  this->updateHarmonics();

  // I found that the initial DC offset could be minimized with an
  // initial state setting as given below.  This initialization should
  // only happen before starting the oscillator for the first time
  // (but after setting the frequency and number of harmonics).  I
  // struggled a bit to decide where best to put this and finally
  // settled on here.  In general, the user shouldn't be messing with
  // the number of harmonics once the oscillator is running because
  // this is automatically taken care of in the setFrequency()
  // function.  (GPS - 1 October 2005)
  state_ = -0.5 * a_;
}

void BlitSaw :: updateHarmonics( void )
{
  if ( nHarmonics_ <= 0 ) {
    unsigned int maxHarmonics = (unsigned int) floor( 0.5 * p_ );
    m_ = 2 * maxHarmonics + 1;
  }
  else
    m_ = 2 * nHarmonics_ + 1;

  a_ = m_ / p_;
}

} // stk namespace